check_license: Work if third_party doesn't exist.

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>

.github/travis/check_license.sh

diff --git a/.github/travis/check_license.sh b/.github/travis/check_license.sh
index 192849a..c8417e1 100755
--- a/.github/travis/check_license.sh
+++ b/.github/travis/check_license.sh
@@ -39,6 +39,13 @@
     exit 1
 fi
 
+THIRD_PARTY_DIRS=$(shopt -s nullglob; echo third_party/*)
+ERROR_NO_LICENSE=""
+
+if [ -z "$THIRD_PARTY_DIRS" ]; then
+	exit 0
+fi
+
 echo
 echo "==========================="
 echo "Check third party LICENSE"
@@ -53,9 +60,6 @@
     done
 }
 
-THIRD_PARTY_DIRS=`ls -d third_party/*`
-ERROR_NO_LICENSE=""
-
 for dir in $THIRD_PARTY_DIRS; do
     # Checks if we are not in a submodule
     if check_if_submodule $dir; then